Synthesis Gas. James G. Speight
In fact, some (or all) of the following processing steps will be required: (i) pretreatment of the feedstock, (ii) primary gasification of the feedstock, (iii) secondary gasification of the carbonaceous residue from the primary gasifier, (iv) removal of carbon dioxide, hydrogen sulfide, and other acid gases, (v) shift conversion for adjustment of the carbon monoxide/hydrogen mole ratio to the desired ratio, and (vi) catalytic methanation of the carbon monoxide/hydrogen mixture to form methane.
Most notable effects in the physical chemistry of the gasification process are those effects due to the chemical character of the feedstock as well as the physical composition of the feedstock (Speight, 2011a, 2013). In more general terms of the character of the feedstock, gasification technologies generally require some initial processing of the feedstock with the type and degree of pretreatment a function of the process and/or the type of feedstock. For example, the Lurgi process will accept lump feedstock (1 in., 25 mm, to 28 mesh), but it must be non-caking with the fines removed – feedstock that show caking or agglomerating tendencies form a plastic mass in the bottom of a gasifier and subsequently plug up the system thereby markedly reducing process efficiency. Thus, some attempt to reduce caking tendencies is necessary and can involve preliminary partial oxidation of the feedstock thereby destroying the caking properties.
Another factor, often presented as very general rule of thumb, is that optimum gas yields and gas quality are obtained at operating temperatures of approximately 595 to 650oC (1100 to 1200oF). A gaseous product with a higher heat content (BTU/ft.3) can be obtained at lower system temperatures but the overall yield of gas (determined as the fuel-to-gas ratio) is reduced by the unburned char fraction.
The major difference between combustion and gasification from the point of view of the chemistry involved is that combustion takes place under oxidizing conditions, while gasification occurs under reducing conditions. In the gasification process, the feedstock (in the presence of steam and oxygen at high temperature and moderate pressure) is converted to a mixture of product gases. In the initial stages of gasification, the rising temperature of the feedstock initiates devolatilization of the feedstock and the breaking of weaker chemical bonds to yield tar, oil, volatile species, and hydrocarbon gases. These products generally react further to form hydrogen, carbon monoxide, and carbon dioxide. The fixed carbon that remains after devolatilization reacts with oxygen, steam, carbon dioxide, and hydrogen.
Depending on the gasifier technology employed and the operating conditions, significant quantities of water, carbon dioxide, and methane can be present in the product gas, as well as a number of minor and trace components. Under the reducing conditions in the gasifier, most of the sulfur in the fuel sulfur is converted to hydrogen sulfide (H2S) as well as to smaller yields of carbonyl sulfide (COS). Organically bound nitrogen in the feedstock is generally (but not always) converted to gaseous nitrogen (N2) – some ammonia (NH3) and a small amount of hydrogen cyanide (HCN) are also formed. Any chlorine in the feedstock (such as coal) is converted to hydrogen chloride (HCl) with some chlorine present in the particulate matter (fly ash). Trace elements, such as mercury and arsenic, are released during gasification and partition among the different phases, such as fly ash, bottom ash, slag, and product gas.
Fuels for gasification reactors differ significantly in chemical properties, physical properties, and morphological properties and, hence, require different reactor design and operation. It is for this reason that, during more than a century of gasification experience, a large number of different gasifiers has been developed – each reactor designed to accommodate the specific properties of a typical fuel or range of fuels. In short, the gasification reactor that is designed to accommodate all (or most) types of fuels does not exist.
However, before choosing a gasifier for any individual fuel it is important to ensure that the fuel meets the requirements of the gasifier or that it can be treated to meet these requirements. Practical tests are needed if the fuel has not previously been successfully gasified. In other words the fuel must match the gasifier and the gasifier must match the fuel.
2.6 Products
The original concept of the gasification process was to produce a fuel gas for use in homes (including street lighting) and industrial operations. Thus, the gasification of carbonaceous residues is generally aimed to feedstock conversion to gaseous products. In fact, gasification offers one of the most versatile methods (with a reduced environmental impact with respect to combustion) to convert carbonaceous feedstocks into electricity, hydrogen, and other valuable energy products. Depending on the previously described type of gasifier (e.g. air-blown, enriched oxygen-blown) and the operating conditions, gasification can be used to produce a fuel gas that is suitable for several applications.
Gasification agents are typically air, oxygen-enriched air or oxygen and the products of the combustion or gasification oxidation reaction change significantly as the oxygen-to-fuel ratio changes from combustion to gasification conditions, which are dependent upon gasifier design and operation (Luque and Speight, 2015). The mixture under gasifying conditions is fuel-rich and there is not enough oxygen to effect complete conversion of the feedstock, in terms of gas quality. As a result, the feedstock carbon reacts to produce carbon instead of carbon dioxide and the feedstock hydrogen is converted to hydrogen rather than to water. Thus, the quantity and quality of the gas generated in a gasification reactor is influenced not only by the feedstock characteristics but predominantly by the gasifier type and configuration well as by the amount of air, oxygen or steam introduced into the system, which is also influence by the configuration of the gasifier.
At the same time, the fate of the nitrogen and sulfur in the fuel is also dictated by oxygen availability (i.e., the configuration of the gasification reactor). The nitrogen and sulfur in a gasification process has important and environmental consequences. Instead of being converted to the respective oxides, the fuel-bound nitrogen is predominantly converted to molecular nitrogen (N2) and hydrogen cyanide (HCN) while the sulfur in the fuel produces hydrogen cyanide (HCN) and carbonyl sulfide (COS).
Steam is sometimes added for temperature control, heating value enhancement or to permit the use of external heat (allothermal gasification). The major chemical reactions break and oxidize hydrocarbon derivatives to give a product gas of carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), and water (H2O). Other important components include hydrogen sulfide (H2S), various compounds of sulfur and carbon, ammonia, low molecular weight hydrocarbon derivatives, and tar.
As a very general rule of thumb, optimum gas yields and gas quality are obtained at operating temperatures of approximately 595 to 650oC (1100 to 1200oF). A gaseous product with a higher heat content (BTU/ft.3) can be obtained at lower system temperatures but the overall yield of gas (determined as the fuel-to-gas ratio) is reduced by the unburned portion of the feedstock, which usually appears as char.
Gasification for electric power generation enables the use of a common technology in modern gas-fired power plants (combined cycle) to recover more of the energy released by burning the fuel. The use of these two types of turbines in the combined cycle system involves (i) a combustion turbine and (ii) a steam turbine. The increased efficiency of the combined cycle for electrical power generation results in a 50% v/v decrease in carbon dioxide emissions compared to conventional coal plants. Gasification units could be modified to further reduce their climate change impact because a large part of the carbon dioxide generated can be separated from the other product gas before combustion (for example carbon dioxide can be separated/sequestered from gaseous byproducts by using adsorbents (e.g., MOFs) to prevent its release to the atmosphere).
Gasification has also been considered for many years as an alternative to combustion of solid or liquid fuels. Gaseous mixtures are simpler to clean as compared to solid or high-viscosity liquid fuels. Cleaned gases can be used in internal combustion-based power plants that would suffer from severe fouling or corrosion if solid or low quality liquid fuels were burned inside them.
In fact, the hot synthesis gas produced by gasification of carbonaceous feedstocks can then be processed to remove sulfur compounds, mercury, and particulate matter prior to its